This invention relates to a method for manufacturing tubing and, more particularly, to an improved method and mandrel plug for the production of cold drawn tubes having varying diameters.
Cold drawing, as used in the production of tubular products, may be generally characterized as the reduction of the diameter and wall thickness of a tube shell or hollow by drawing it through a fixed cold reduction die. Several processes of cold drawing tubes are conventionally utilized.
Cold drawing a tubular hollow through a die without an internal mandrel, for example, is known as sinking or sink drawing. In a sinking process, only the diameter of the tube hollow is reduced. The wall thickness is essentially maintained but may be slightly increased or decreased depending upon the ratio of the wall thickness to the tube diameter. A sink drawing technique is utilized if the outside diameter must be only slightly reduced to achieve a desired size and tolerance, or if it is not otherwise necessary to reduce the wall thickness.
Simultaneous wall thickness and tube diameter reductions may be achieved by several drawing methods including drawing with a stationary mandrel, drawing with a floating plug mandrel, or drawing with a moving mandrel.
In drawing with a stationary mandrel, a cylindrical mandrel plug is rigidly coupled to an elongated mandrel rod of smaller cross section. The mandrel plug is fixed in position within a die opening thereby forming an annulus between the cylindrical mandrel plug and die opening. The die opening, typically smaller than the outside diameter of the tube hollow which is to be cold worked, is provided with a tapered entrance or approach leading to a circular die land (bearing zone). A tube hollow is drawn through the annulus reducing its diameter and thickness. The cross section of the tube passing from the die opening is approximately equal to the cross section of the annulus at the die land.
In the floating plug method of tube drawing, the mandrel plug is not fixed to a rod. The floating plug is configured to automatically adjust itself to the correct operating position during the drawing operation. Typically, the floating plug has a contour that includes a conical portion and cylindrical portion. The cylindrical portion of the plug acts in a manner similar to a stationary mandrel. The angles of the conical portion of the plug and the tapered entrance of the die are such that as the tube hollow is drawn through the die, frictional forces arising between the plug and the inside surface of the tube hollow, cause the plug to automatically adjust or float, and position the plug to form an annulus of constant size between the cylindrical portion of the plug and the die land. Long tubes can be cold drawn by this technique, since the weight and length of a connecting mandrel rod are not a consideration.
Drawing with a moving mandrel encompasses the use of a long internally disposed rod which moves with a circumscribed tube hollow through the die. Drawing with a moving mandrel is usually restricted to the production of small diameter or thin walled tubing wherein the rod which would have to be attached to a stationary mandrel would be too thin to maintain its structural integrity or where friction between a fixed or floating plug might damage the thin walled tubing.
Cold drawing is often employed to produce tubing with mechanical properties or qualities which cannot be obtained with standard hot rolling operations. Cold drawing is utilized to form tubes with thinner walls and improved dimensional tolerances; to achieve greater mechanical properties including yield strength, ultimate tensile strength and hardness; and, to produce non-standard sizes, shapes or sections.
The quality of the tube hollows has considerable bearing on the quality of the finished tube. Therefore, several preliminary operations are performed. First, the tube hollow is cut to the proper length for drawing. One end of the tube hollow is then pointed or swaged. This end will subsequently be gripped by a mechanism, typically a draw carriage, which will actually draw the tube hollow through the die. The tube hollow may next be annealed to facilitate the cold working by softening, to add ductility or to develop the proper metal microstructure. The tube hollow is pickled in dilute acid solutions to remove scale and surface dirt. After pickling, the tube hollow is carefully inspected to assure the absence of off-mill imperfections or defects such as seams, slivers and mill marks that can have deleterious effects on the finished tube. Any defects found are removed prior to the drawing operations. The tube hollow is then lubricated to minimize the frictional forces which will arise as it is drawn through the die. Multiple pass drawing operations, wherein several draw passes are required to produce the desired product, generally necessitate intermediate annealing and repetition of a number of these preparatory steps.
Tubular wall reductions achievable in a draw pass have practical limitations. The amount of wall reduction that a tube can physically withstand per draw pass without damage is a function of its ductility. However, the amount of reduction of area in one pass should generally not exceed thirty-five percent.
Shaped tubes and variable section--stepped internal diameter--tubes are employed in engineering for manufacturing parts for different industries. A basic purpose for using variable section tubing is to produce lighter tubular structures while maintaining strength and saving metal. Tubes having stepped diameters may be fabricated so as to minimize subsequent machining operations, for instance, where a tubular product is to be subjected to a final internal boring to achieve a precision tolerance or dimension. Stepped tubes may also be useful where varying or extra wall thickness is necessary for subsequent operations such as bending, for connections to tubes of different wall thickness or diameter, or to a header or tube sheet having uniform tube holes, or for maximizing the number of tubes in a given space.
In order to form a stepped cold finished tube, it is generally necessary to subject the wall of the tube hollow which will constitute the thinner section to several cold drawing passes. Typically, a portion of the tube hollow is sunk drawn to a predetermined length substantially maintaining the initial wall thickness. A stationary mandrel is positioned within the hollow. The hollow is passed further on through the die, reducing the wall of a succeeding portion of the hollow, and producing a step in internal diameter at the interface of the original and reduced cross-section portions. In order to further reduce the walls the hollows must be annealed and subjected to a number of preparatory steps, such as described, prior to further drawing. Further reduction of the tube hollow to produce a finished tube may be accomplished by repeating the described steps.
Significant disadvantages are inherent in this procedure. The repeated outside diameter reduction by sinking (without a mandrel) of the heavy wall part of the tube tends to produce very small but significant defects on the inside surface of the sunk part of the tube. In the presence of residual or service stresses in the walls of such tubes, such as might result from cold sinking, from certain heat treatments, or from the presence of an internal fluid under pressure, these small defects can cause local concentrations of stress sufficient to cause splitting of the tube wall, either during manufacture of the tube or later in service. Furthermore, accurate repositioning of the mandrel plug at the exact time of arrival of the preliminary step upon subsequent passes is quite difficult and is not conducive to producing a high quality product or maintaining high productivity.